Implantable stimulation devices generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. patent application Ser. No. 11/177,503, filed Jul. 8, 2005, which is incorporated herein by reference in its entirety.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. An SCS system typically includes an Implantable Pulse Generator (IPG) or Radio-Frequency (RF) transmitter and receiver, electrodes, at least one electrode lead, and, optionally, at least one electrode lead extension. The electrodes, which reside on a distal end of the electrode lead, are typically implanted along the dura of the spinal cord, and the IPG or RF transmitter generates electrical pulses that are delivered through the electrodes to the nerve fibers within the spinal column. Individual electrode contacts (the “electrodes”) are arranged in a desired pattern and spacing to create an electrode array. Individual wires within one or more electrode leads connect with each electrode in the array. The electrode lead(s) exit the spinal column and generally attach to one or more electrode lead extensions. The electrode lead extensions, in turn, are typically tunneled around the torso of the patient to a subcutaneous pocket where the EPG or RF transceiver is implanted. Alternatively, the electrode lead may directly connect with the IPG or RF transceiver. For examples of other SCS systems and other stimulation systems, see U.S. Pat. Nos. 3,646,940 and 3,822,708, which are hereby incorporated by reference in their entireties. Of course, implantable pulse generators are active devices requiring energy for operation, such as is provided by an implanted battery or an external power source.
FIGS. 1-3 introduce various components of an exemplary SCS system, although further details will be explained more fully later. As particularly relevant to the present discussion, the SCS components comprise implantable components 10 (i.e., components implantable or implanted into a patient requiring therapy) and external components 20 (i.e., components external to the patient but which work in conjunction with the internal components 10). As seen in FIG. 1, the implantable components 10 include an implantable pulse generator (IPG) 100, which may comprise a rechargeable, multi-channel, telemetry-controlled, pulse generator. The external components 20 include a remote control 202, otherwise known as a hand-held programmer (HHP) 202, which may be used to control the EPG 100 via a suitable non-invasive communications link 201, e.g., an RF link. Such control allows the IPG 100 to be turned on or off, and generally allows stimulation parameters, e.g., pulse amplitude, width, and rate, to be set within prescribed limits. Detailed, system-level programming of the IPG 100 may additionally be accomplished through the use of an external clinician's programmer (CP) 204, which may also be hand-held and which may be coupled to the IPG 100 directly via an RF link 201a or indirectly using the HHP 202 as an intermediary. These RF links 201, 201a are preferably two-way links that can be used to send data to (i.e., control) the IPG 100, or to receive data from the IPG 100.
Such RF telemetry between the HHP 202 or CP 204 and the IPG 100 is supported via circuitry in the IPG 100, as shown in FIG. 3. Among other components and circuitry which will be described in further detail later, the IPG 100 comprises RF-telemetry circuitry 172, which receives RF telemetry data from the external components 20 (such as desired IPG operating parameters) and which sends RF telemetry data to the external components 20 (e.g., to allow the IPG 100's operating parameters to be verified, to allow the IPG 100's identification number to be reported, etc.).
In recognition of the fact that the RF telemetry through links 201 and 201a would generally comprise use of a modulated carrier, RF-telemetry circuitry 172 would preferably include demodulator circuitry 262. Exemplary frequency demodulation circuitry useable in an IPG 100, as well as other components of the RF-telemetry circuitry 172, is shown in FIG. 5. What is shown for simplicity is an analog FM demodulation circuit, but one skilled in the art will recognize that it can be implemented digitally as well, and preferably would be implemented digitally in an implantable stimulator application. (In a digital implementation, some of the circuit elements shown would not be used, such as the LC circuit and mixer).
The operation of the demodulation circuitry is known to one skilled in the art, and hence is only briefly described. Essentially, data is sent to the demodulation circuitry (e.g., via RF links 201, 201a) as a sequence of bits represented by a variance in frequency (121 kHz, 129 kHz) from a center carrier frequency (fc=125 kHz). After passing the received signal through a band pass filter to remove frequencies outside of the frequency range of interest, a phase shift (φ) is induced in the received signal via an LC circuit for example, in which the phase shift is a function of the frequency of the received signal. By mixing the phase shifted signal with the original received signal, and sending the result through a low pass filter to remove high-frequency components, a voltage (proportional to ½ cos(φ)) is generated which is compared to a threshold to determine whether the received signal comprised a 121 kHz signal (a logical ‘0’) or a 129 kHz signal (a logical ‘1’). As noted earlier, digital demodulation is logical in an implantable medical device application, and could for example comprise use of the QFAST RF protocol, which supports bi-directional telemetry at, e.g., 8 Kbits/second. (QFAST stands for “Quadrature Fast Acquisition Spread Spectrum Technique,” and represents a known and viable approach for modulating and demodulating data).
Regardless of whether an IPG 100 is powered by a non-rechargeable battery, or is powered by a battery rechargeable via an RF energy source (e.g., charger 208, FIG. 1), or is solely powered via an RF energy source, power consumption in an IPG is preferably kept to a minimum. For example, in the case of an IPG with a rechargeable battery, lower power consumption equates to longer periods in which the EPG can be used to provide stimulation between charges.
Telemetry procedures such as those just described can affect power consumption in an IPG 100. An IPG 100, regardless of whether it is currently providing stimulus to the patient in which it is implanted, needs to be ready for the possibility that an external component 20 (e.g., HHP 202 or CP 204) wishes to communicate with it, and hence must “listen” for relevant telemetry from the external component. Because power consumption in an external component is generally less critical (because it is external to the patient; because it can be plugged in or easily provided with fresh batteries, etc.), the external component can repeatedly broadcast its desire to communicate with the IPG, and then wait for the IPG to respond before sending its command to the IPG. In other words, the external component may broadcast nearly continually, aside from short periods to listen for a response from the IPG. This can be thought of as a “handshaking” or “wake up” procedure initiated by the external component, in which a wake-up signal is broadcast by the external component.
But such a handshaking approach necessitates that the IPG 100, and specifically the RF-telemetry circuitry 172, be powered by the IPG 100, because only when such circuitry 172 is powered can the IPG 100 recognize the wake-up signal from the external component and in turn telemeter an acknowledgment back to the external component. Ideally therefore, the RF-telemetry circuitry 172 would be powered by the IPG 100 at all times so that the IPG 100 could recognize an external component's wake-up signal immediately. But this is not practical, especially considering the relative infrequency that an external component might wish to communicate with an IGP 100. In short, keeping the RF-telemetry circuitry 172 powered at all times is not an efficient solution, as it drains too much power from the IPG 100.
In recognition of this fact, a procedure may be employed in which the RF-telemetry circuitry 172 is only occasionally powered by the IPG 100, for example, once every few seconds for a “window” of time comprising a number of milliseconds. While such an approach sacrifices immediacy in the IPG 100's recognition of the broadcast wake-up signal, it allows the RF-telemetry circuitry 172 to be powered only a fraction of the time, i.e., during the several millisecond “power-on window.” This saves power, while still allowing the external component's broadcast wake-up signal to be eventually recognized and responded to by the IPG 100.
But even this procedure is potentially wasteful of power in the IPG. This is because the power-on window generally needs to be on for the entire length of time that it would take to receive a valid wake-up signal from the external component. In this regard, recognize that a broadcasted wake-up signal from an external component would generally comprise a serial stream of binary bits. Assume for example that the RF-telemetry circuitry 172 must recognize (i.e., demodulate) a certain number of bits in this sequence before acknowledging the wake-up signal and responding in kind with an acknowledgment. Such a scheme would dictate that the power-on window be active for the entirety of the time it would take to receive the number of bits that the IPG 100 must recognize as the wake-up signal. For example, suppose each bit in the sequence is broadcast by the external component for approximately 250 microseconds, and that the IPG 100 is looking for a particular 12-bit sequence as the wake-up signal. This means it would take at least 3 milliseconds (250 microseconds times 12) for the IPG 100 to understand receipt of the entire wake-up signal, and hence that the RF-telemetry circuitry 172 must be powered for at least that long. Indeed, depending on when the IPG 100 powers on the RF-telemetry circuitry 172 relative to the start of the broadcasted wake-up signal, the IPG 100 might well need to keep the RF-telemetry circuitry 172 powered on for longer than 3 milliseconds to understand the beginning of the sequence and to then fully receive it.
Having the IPG 100 power on the RF-telemetry circuit 172 window for even a mere 3 milliseconds every second or so is still significant in terms of the power that is consumed by the IPG 100, particularly given that handshaking from the external components would occur relatively rarely. Accordingly, the implantable stimulator art, or more specifically the IPG or SCS system art, would benefit from telemetry techniques for handshaking between an external component and an IPG that are less wasteful of power. Such solutions are provided herein.